The detachment of a thin film from the remainder of the a source substrate is based on the observation that an implantation of chemical species in the source substrate may induce the formation of a zone of defects at a given depth. These defects may be micro-bubbles and/or platelets and/or micro-cavities and/or dislocation loops and/or other crystalline defects, disrupting the crystalline quality of the material, of which the nature, the density and the size are strongly dependent on the species implanted as well as on the nature of the source substrate. A heat-treatment may then be applied to enable the development of specific defects present in the weakened zone, which will enable the detachment of the thin film from the source substrate to be obtained later. This has in particular been described in U.S. Pat. No. 5,374,564 and developments thereof, such as described in U.S. Pat. No. 5,374,564.
The implantation step has been the subject of numerous research projects and studies in the specific field of SOI. In that context the problem to resolve is generally to reduce the implantation doses in order, on the one hand, to reduce the costs of manufacture by reducing the time of use of the machine, and, on the other hand, from a technological point of view, to reduce the zone damaged by the implantation.
Thus, for example, Agarwal et al (1997) gave an account, in “Efficient production of silicon-on-insulator films by co-implantation of He+ with H+”, Applied Physics Letters, Volume 72, Number 9, 2 Mar. 1998, of trials carried out by applying ions of two types, that is to say a co-implantation of the two species hydrogen and helium, in a silicon substrate. The authors specify that the implantation profiles of the two implanted species must be localized at the same depth, around which the concentration in implanted species is maximum and it is at that location that propagation of the splitting will be induced. The authors teach that the order of implantation of the two implanted species is important: hydrogen must be implanted first, helium second. They comment that it is thus possible to reduce the total implanted dose by a factor of the order of three in relation to the use of each species alone.
More particularly, this document discloses trials with low doses (7.5×1015 H+/cm2 and 1×1016 He/cm2; or 1×1016 H+/cm2 and 1×1016 He/cm2) on the SOI. The detachment is then obtained at a usual temperature (500° C.) with a low total implanted dose.
It may be noted that this document describes an experimental approach and gives little importance to the target substrate.
Similar teachings may be found in U.S. Patent Publication No. 2002/0025604 which concerns a low-temperature semiconductor layering and three-dimensional electronic circuits using the layering. Such layering method includes several steps. First hydrogen and then helium are implanted at doses between 1×1016/cm2 and 4×1016/cm2 with a range that is close to each other. Implanted wafer is then bonded to another wafer. The bonded wafers are then annealed at low temperature between 200–250° C. for 1 to 48 hours and annealed at 400–600° C. for 1 to 10 minutes so that a portion of the wafer is detached. This document concerns layering silicon on a silicon substrate.
The problem is posed in a very different manner in the case of heterostructures, that is to say in the case in which the materials of the source and target substrates are different. In this case, one of the major technological problems encountered is the presence of a field of very high stress in the various layers in contact, during the heat-treatment such as that during which the detachment of the thin film from the remainder of the source substrate occurs: this stress field is due to the difference in coefficients of thermal expansion between the various materials brought into contact.
Thus, in the case of substrates with different thermal expansion coefficients (heterostructure), it is important to manage to achieve the detachment at a lower temperature than the critical temperature at which the heterostructure will be degraded on account of the aforementioned mechanical stresses. This degradation may typically result in the breakage of one or both substrates brought into contact and/or in the substrates becoming unbonded at the bonding interface. For example, in a heterostructure comprising a implanted substrate of Si bonded to a fused silica substrate, the detachment of the Si layer on the fused silica substrate is accompanied by the breakage of the substrates if the heterostructure is subjected to a heat-treatment at 500° C. It is thus desirable to reduce the heat-treatment temperature to avoid the breakage or any damage of the heterostructure (and/or of the two substrates obtained after detachment) and to maintain a good quality for the transferred layer.
The same need to be able to use a relatively low detachment temperature is met when compounds are formed in one of the substrates (for example in the future thin film) and are liable to be degraded during a heat-treatment which is too aggressive.
One way to reduce the temperature of obtainment of the detachment is to “play” with the implantation conditions. For example, an excess dose of the implanted species makes it possible to reduce the thermal budget for detachment, thermal budget being understood to mean the pair Length of heat-treatment/Temperature of heat-treatment.
Bruel et al. (ECS Spring Meeting 1999) have thus shown that if the source substrate is a wafer of silicon, a dose of hydrogen ions implanted at 1×1017 H/cm2, instead of at 5.5×1016 H/cm2, makes it possible, for a limited duration of heat-treatment of a few hours, to reduce the detachment temperature from 425° C. to 280° C.
This approach, although reducing the thermal budget for detachment, uses an implantation at high dose which may represent a significant drawback from an industrial point of view (high cost). Furthermore, it is of note that, due to the high implanted dose, the disrupted zone (comprising defects related to the implantation) at the surface of the transferred layer is thicker and the later processing operations necessary to eliminate that disrupted superficial zone may be more restrictive (greater removal of material, corresponding to more costly processing and potentially increasing the risks of lack of homogeneity of the thickness of the transferred layer).
Another idea to reduce the temperature of detachment is described in U.S. Pat. No. 5,877,070 to Gosele et al. It consists in implanting firstly an element involving the formation of hydrogen traps (in particular boron, carbon, phosphorus, nitrogen, arsenic or fluorine, that is to say elements of considerable size) then implanting the hydrogen in the source substrate, and in carrying out an operation of prior annealing before bonding of the source and target substrates. According to the inventors this enables the detachment temperature to be reduced by 50% in comparison with the case of implantation of H+ alone. The invention relies on two steps: co-implantation (in which the hydrogen is introduced secondly) and a pre-annealing of the source substrate.
The invention relates to a method of detaching a thin film from a source substrate (for example fixed beforehand onto a target substrate, advantageously of a different material to that of the source substrate), which does not require implantation doses that are too high nor the annealing of the source substrate after implantation (and, where the case arises, before its bonding onto the target substrate), while permitting the detachment at a temperature sufficiently low not to induce, when the source substrate is fixed to a target substrate and when their coefficients of thermal expansion are different, prohibitive mechanical stresses on the heterostructure constituted by the two substrates, and/or not to risk degrading components which may have been formed on one of the substrates before detachment.